最近有許多小夥伴後臺聯絡我，說目前想要學習Python，但是沒有一份很好的資料入門。一方面的確現在市面上Python的資料過多，導致新手會不知如何選擇，另一個問題很多資料內容也很雜，從1+1到深度學習都包括，純粹關注Python本身語法的優質教材並不太多。

剛好我最近看到一份不錯的英文Python入門資料，我將它做了一些整理和翻譯寫下了本文。這份資料非常純粹，只有Python的基礎語法，專門針對想要學習Python的小白。

註釋

Python中用#表示單行註釋，#之後的同行的內容都會被註釋掉。

# Python中單行註釋用#表示，#之後同行字元全部認為被註釋。

使用三個連續的雙引號表示多行註釋，兩個多行註釋標識之間內容會被視作是註釋。

""" 與之對應的是多行註釋
    用三個雙引號表示，這兩段雙引號當中的內容都會被視作是註釋
"""

基礎變數型別與操作符

Python當中的數字定義和其他語言一樣：

#獲得一個整數
3
# 獲得一個浮點數
10.0

我們分別使用+, -, *, /表示加減乘除四則運算子。

1 + 1   # => 2
8 - 1   # => 7
10 * 2  # => 20
35 / 5  # => 7.0

這裡要注意的是，在Python2當中，10/3這個操作會得到3，而不是3.33333。因為除數和被除數都是整數，所以Python會自動執行整數的計算，幫我們把得到的商取整。如果是10.0 / 3，就會得到3.33333。目前Python2已經不再維護了，可以不用關心其中的細節。

但問題是Python是一個弱型別的語言，如果我們在一個函式當中得到兩個變數，是無法直接判斷它們的型別的。這就導致了同樣的計算符可能會得到不同的結果，這非常蛋疼。以至於程式設計師在運算除法的時候，往往都需要手工加上型別轉化符，將被除數轉成浮點數。

在Python3當中撥亂反正，修正了這個問題，即使是兩個整數相除，並且可以整除的情況下，得到的結果也一定是浮點數。

如果我們想要得到整數，我們可以這麼操作：

5 // 3       # => 1
-5 // 3      # => -2
5.0 // 3.0   # => 1.0 # works on floats too
-5.0 // 3.0  # => -2.0
 

兩個除號表示取整除，Python會為我們保留去除餘數的結果。

除了取整除操作之外還有取餘數操作，數學上稱為取模，Python中用%表示。

# Modulo operation
7 % 3  # => 1

Python中支援乘方運算，我們可以不用呼叫額外的函式，而使用**符號來完成：

# Exponentiation (x**y, x to the yth power)
2**3  # => 8

當運算比較複雜的時候，我們可以用括號來強制改變運算順序。

# Enforce precedence with parentheses
1 + 3 * 2  # => 7
(1 + 3) * 2  # => 8

邏輯運算

Python中用首字母大寫的True和False表示真和假。

True  # => True
False  # => False

用and表示與操作，or表示或操作，not表示非操作。而不是C++或者是Java當中的&&, || 和！。

# negate with not
not True   # => False
not False  # => True

# Boolean Operators
# Note "and" and "or" are case-sensitive
True and False  # => False
False or True   # => True

在Python底層，True和False其實是1和0，所以如果我們執行以下操作，是不會報錯的，但是在邏輯上毫無意義。

# True and False are actually 1 and 0 but with different keywords
True + True # => 2
True * 8    # => 8
False - 5   # => -5

我們用==判斷相等的操作，可以看出來True==1， False == 0.

# Comparison operators look at the numerical value of True and False
0 == False  # => True
1 == True   # => True
2 == True   # => False
-5 != False # => True

我們要小心Python當中的bool()這個函式，它並不是轉成bool型別的意思。如果我們執行這個函式，那麼只有0會被視作是False，其他所有數值都是True：

bool(0)     # => False
bool(4)     # => True
bool(-6)    # => True
0 and 2     # => 0
-5 or 0     # => -5

Python中用==判斷相等，>表示大於，>=表示大於等於， <表示小於，<=表示小於等於，!=表示不等。

# Equality is ==
1 == 1  # => True
2 == 1  # => False

# Inequality is !=
1 != 1  # => False
2 != 1  # => True

# More comparisons
1 < 10  # => True
1 > 10  # => False
2 <= 2  # => True
2 >= 2  # => True

我們可以用and和or拼裝各個邏輯運算：

# Seeing whether a value is in a range
1 < 2 and 2 < 3  # => True
2 < 3 and 3 < 2  # => False
# Chaining makes this look nicer
1 < 2 < 3  # => True
2 < 3 < 2  # => False

注意not，and，or之間的優先順序，其中not > and > or。如果分不清楚的話，可以用括號強行改變執行順序。

list和字串

關於list的判斷，我們常用的判斷有兩種，一種是剛才介紹的==，還有一種是is。我們有時候也會簡單實用is來判斷，那麼這兩者有什麼區別呢？我們來看下面的例子：

a = [1, 2, 3, 4]  # Point a at a new list, [1, 2, 3, 4]
b = a             # Point b at what a is pointing to
b is a            # => True, a and b refer to the same object
b == a            # => True, a's and b's objects are equal
b = [1, 2, 3, 4]  # Point b at a new list, [1, 2, 3, 4]
b is a            # => False, a and b do not refer to the same object
b == a            # => True, a's and b's objects are equal

Python是全引用的語言，其中的物件都使用引用來表示。is判斷的就是兩個引用是否指向同一個物件，而==則是判斷兩個引用指向的具體內容是否相等。舉個例子，如果我們把引用比喻成地址的話，is就是判斷兩個變數的是否指向同一個地址，比如說都是沿河東路XX號。而==則是判斷這兩個地址的收件人是否都叫張三。

顯然，住在同一個地址的人一定都叫張三，但是住在不同地址的兩個人也可以都叫張三，也可以叫不同的名字。所以如果a is b，那麼a == b一定成立，反之則不然。

Python當中對字串的限制比較鬆，雙引號和單引號都可以表示字串，看個人喜好使用單引號或者是雙引號。我個人比較喜歡單引號，因為寫起來方便。

字串也支援+操作，表示兩個字串相連。除此之外，我們把兩個字串寫在一起，即使沒有+，Python也會為我們拼接：

# Strings are created with " or '
"This is a string."
'This is also a string.'

# Strings can be added too! But try not to do this.
"Hello " + "world!"  # => "Hello world!"
# String literals (but not variables) can be concatenated without using '+'
"Hello " "world!"    # => "Hello world!"

我們可以使用[]來查詢字串當中某個位置的字元，用len來計算字串的長度。

# A string can be treated like a list of characters
"This is a string"[0]  # => 'T'

# You can find the length of a string
len("This is a string")  # => 16

我們可以在字串前面加上f表示格式操作，並且在格式操作當中也支援運算，比如可以巢狀上len函式等。不過要注意，只有Python3.6以上的版本支援f操作。

# You can also format using f-strings or formatted string literals (in Python 3.6+)
name = "Reiko"
f"She said her name is {name}." # => "She said her name is Reiko"
# You can basically put any Python statement inside the braces and it will be output in the string.
f"{name} is {len(name)} characters long." # => "Reiko is 5 characters long."

最後是None的判斷，在Python當中None也是一個物件，所有為None的變數都會指向這個物件。根據我們前面所說的，既然所有的None都指向同一個地址，我們需要判斷一個變數是否是None的時候，可以使用is來進行判斷，當然用==也是可以的，不過我們通常使用is。

# None is an object
None  # => None

# Don't use the equality "==" symbol to compare objects to None
# Use "is" instead. This checks for equality of object identity.
"etc" is None  # => False
None is None   # => True

理解了None之後，我們再回到之前介紹過的bool()函式，它的用途其實就是判斷值是否是空。所有型別的預設空值會被返回False，否則都是True。比如0，""，[], {}, ()等。

# None, 0, and empty strings/lists/dicts/tuples all evaluate to False.
# All other values are True
bool(None)# => False
bool(0)   # => False
bool("")  # => False
bool([])  # => False
bool({})  # => False
bool(())  # => False

除了上面這些值以外的所有值傳入都會得到True。

變數與集合

輸入輸出

Python當中的標準輸入輸出是input和print。

print會輸出一個字串，如果傳入的不是字串會自動呼叫__str__方法轉成字串進行輸出。預設輸出會自動換行，如果想要以不同的字元結尾代替換行，可以傳入end引數：

# Python has a print function
print("I'm Python. Nice to meet you!")  # => I'm Python. Nice to meet you!

# By default the print function also prints out a newline at the end.
# Use the optional argument end to change the end string.
print("Hello, World", end="!")  # => Hello, World!

使用input時，Python會在命令列接收一行字串作為輸入。可以在input當中傳入字串，會被當成提示輸出：

# Simple way to get input data from console
input_string_var = input("Enter some data: ") # Returns the data as a string
# Note: In earlier versions of Python, input() method was named as raw_input()

變數

Python中宣告物件不需要帶上型別，直接賦值即可，Python會自動關聯型別，如果我們使用之前沒有宣告過的變數則會出發NameError異常。

# There are no declarations, only assignments.
# Convention is to use lower_case_with_underscores
some_var = 5
some_var  # => 5

# Accessing a previously unassigned variable is an exception.
# See Control Flow to learn more about exception handling.
some_unknown_var  # Raises a NameError

Python支援三元表示式，但是語法和C++不同，使用if else結構，寫成：

# if can be used as an expression
# Equivalent of C's '?:' ternary operator
"yahoo!" if 3 > 2 else 2  # => "yahoo!"

上段程式碼等價於：

if 3 > 2:
    return 'yahoo'
else:
    return 2

list

Python中用[]表示空的list，我們也可以直接在其中填充元素進行初始化：

# Lists store sequences
li = []
# You can start with a prefilled list
other_li = [4, 5, 6]

使用append和pop可以在list的末尾插入或者刪除元素：

# Add stuff to the end of a list with append
li.append(1)    # li is now [1]
li.append(2)    # li is now [1, 2]
li.append(4)    # li is now [1, 2, 4]
li.append(3)    # li is now [1, 2, 4, 3]
# Remove from the end with pop
li.pop()        # => 3 and li is now [1, 2, 4]
# Let's put it back
li.append(3)    # li is now [1, 2, 4, 3] again.

list可以通過[]加上下標訪問指定位置的元素，如果是負數，則表示倒序訪問。-1表示最後一個元素，-2表示倒數第二個，以此類推。如果訪問的元素超過陣列長度，則會出發IndexError的錯誤。

# Access a list like you would any array
li[0]   # => 1
# Look at the last element
li[-1]  # => 3

# Looking out of bounds is an IndexError
li[4]  # Raises an IndexError

list支援切片操作，所謂的切片則是從原list當中拷貝出指定的一段。我們用start: end的格式來獲取切片，注意，這是一個左閉右開區間。如果留空表示全部獲取，我們也可以額外再加入一個引數表示步長，比如[1:5:2]表示從1號位置開始，步長為2獲取元素。得到的結果為[1, 3]。如果步長設定成-1則代表反向遍歷。

# You can look at ranges with slice syntax.
# The start index is included, the end index is not
# (It's a closed/open range for you mathy types.)
li[1:3]   # Return list from index 1 to 3 => [2, 4]
li[2:]    # Return list starting from index 2 => [4, 3]
li[:3]    # Return list from beginning until index 3  => [1, 2, 4]
li[::2]   # Return list selecting every second entry => [1, 4]
li[::-1]  # Return list in reverse order => [3, 4, 2, 1]
# Use any combination of these to make advanced slices
# li[start:end:step]

如果我們要指定一段區間倒序，則前面的start和end也需要反過來，例如我想要獲取[3: 6]區間的倒序，應該寫成[6:3:-1]。

只寫一個:，表示全部拷貝，如果用is判斷拷貝前後的list會得到False。可以使用del刪除指定位置的元素，或者可以使用remove方法。

# Make a one layer deep copy using slices
li2 = li[:]  # => li2 = [1, 2, 4, 3] but (li2 is li) will result in false.

# Remove arbitrary elements from a list with "del"
del li[2]  # li is now [1, 2, 3]

# Remove first occurrence of a value
li.remove(2)  # li is now [1, 3]
li.remove(2)  # Raises a ValueError as 2 is not in the list

insert方法可以指定位置插入元素，index方法可以查詢某個元素第一次出現的下標。

# Insert an element at a specific index
li.insert(1, 2)  # li is now [1, 2, 3] again

# Get the index of the first item found matching the argument
li.index(2)  # => 1
li.index(4)  # Raises a ValueError as 4 is not in the list

list可以進行加法運算，兩個list相加表示list當中的元素合併。等價於使用extend方法：

# You can add lists
# Note: values for li and for other_li are not modified.
li + other_li  # => [1, 2, 3, 4, 5, 6]

# Concatenate lists with "extend()"
li.extend(other_li)  # Now li is [1, 2, 3, 4, 5, 6]

我們想要判斷元素是否在list中出現，可以使用in關鍵字，通過使用len計算list的長度：

# Check for existence in a list with "in"
1 in li  # => True

# Examine the length with "len()"
len(li)  # => 6

tuple

tuple和list非常接近，tuple通過()初始化。和list不同，tuple是不可變物件。也就是說tuple一旦生成不可以改變。如果我們修改tuple，會引發TypeError異常。

# Tuples are like lists but are immutable.
tup = (1, 2, 3)
tup[0]      # => 1
tup[0] = 3  # Raises a TypeError

由於小括號是有改變優先順序的含義，所以我們定義單個元素的tuple，末尾必須加上逗號，否則會被當成是單個元素：

# Note that a tuple of length one has to have a comma after the last element but
# tuples of other lengths, even zero, do not.
type((1))   # => <class 'int'>
type((1,))  # => <class 'tuple'>
type(())    # => <class 'tuple'>

tuple支援list當中絕大部分操作：

# You can do most of the list operations on tuples too
len(tup)         # => 3
tup + (4, 5, 6)  # => (1, 2, 3, 4, 5, 6)
tup[:2]          # => (1, 2)
2 in tup         # => True

我們可以用多個變數來解壓一個tuple：

# You can unpack tuples (or lists) into variables
a, b, c = (1, 2, 3)  # a is now 1, b is now 2 and c is now 3
# You can also do extended unpacking
a, *b, c = (1, 2, 3, 4)  # a is now 1, b is now [2, 3] and c is now 4
# Tuples are created by default if you leave out the parentheses
d, e, f = 4, 5, 6  # tuple 4, 5, 6 is unpacked into variables d, e and f
# respectively such that d = 4, e = 5 and f = 6
# Now look how easy it is to swap two values
e, d = d, e  # d is now 5 and e is now 4

解釋一下這行程式碼：

a, *b, c = (1, 2, 3, 4)  # a is now 1, b is now [2, 3] and c is now 4

我們在b的前面加上了星號，表示這是一個list。所以Python會在將其他變數對應上值的情況下，將剩下的元素都賦值給b。

補充一點，tuple本身雖然是不可變的，但是tuple當中的可變元素是可以改變的。比如我們有這樣一個tuple：

a = (3, [4])

我們雖然不能往a當中新增或者刪除元素，但是a當中含有一個list，我們可以改變這個list型別的元素，這並不會觸發tuple的異常：

a[1].append(0) # 這是合法的

dict

dict也是Python當中經常使用的容器，它等價於C++當中的map，即儲存key和value的鍵值對。我們用{}表示一個dict，用:分隔key和value。

# Dictionaries store mappings from keys to values
empty_dict = {}
# Here is a prefilled dictionary
filled_dict = {"one": 1, "two": 2, "three": 3}

dict的key必須為不可變物件，所以list、set和dict不可以作為另一個dict的key，否則會丟擲異常：

# Note keys for dictionaries have to be immutable types. This is to ensure that
# the key can be converted to a constant hash value for quick look-ups.
# Immutable types include ints, floats, strings, tuples.
invalid_dict = {[1,2,3]: "123"}  # => Raises a TypeError: unhashable type: 'list'
valid_dict = {(1,2,3):[1,2,3]}   # Values can be of any type, however.

我們同樣用[]查詢dict當中的元素，我們傳入key，獲得value，等價於get方法。

# Look up values with []
filled_dict["one"]  # => 1
filled_dict.get('one') #=> 1

我們可以call dict當中的keys和values方法，獲取dict當中的所有key和value的集合，會得到一個list。在Python3.7以下版本當中，返回的結果的順序可能和插入順序不同，在Python3.7及以上版本中，Python會保證返回的順序和插入順序一致：

# Get all keys as an iterable with "keys()". We need to wrap the call in list()
# to turn it into a list. We'll talk about those later.  Note - for Python
# versions <3.7, dictionary key ordering is not guaranteed. Your results might
# not match the example below exactly. However, as of Python 3.7, dictionary
# items maintain the order at which they are inserted into the dictionary.
list(filled_dict.keys())  # => ["three", "two", "one"] in Python <3.7
list(filled_dict.keys())  # => ["one", "two", "three"] in Python 3.7+

# Get all values as an iterable with "values()". Once again we need to wrap it
# in list() to get it out of the iterable. Note - Same as above regarding key
# ordering.
list(filled_dict.values())  # => [3, 2, 1]  in Python <3.7
list(filled_dict.values())  # => [1, 2, 3] in Python 3.7+

我們也可以用in判斷一個key是否在dict當中，注意只能判斷key。

# Check for existence of keys in a dictionary with "in"
"one" in filled_dict  # => True
1 in filled_dict      # => False

如果使用[]查詢不存在的key，會引發KeyError的異常。如果使用get方法則不會引起異常，只會得到一個None：

# Looking up a non-existing key is a KeyError
filled_dict["four"]  # KeyError

# Use "get()" method to avoid the KeyError
filled_dict.get("one")      # => 1
filled_dict.get("four")     # => None
# The get method supports a default argument when the value is missing
filled_dict.get("one", 4)   # => 1
filled_dict.get("four", 4)  # => 4

setdefault方法可以為不存在的key插入一個value，如果key已經存在，則不會覆蓋它：

# "setdefault()" inserts into a dictionary only if the given key isn't present
filled_dict.setdefault("five", 5)  # filled_dict["five"] is set to 5
filled_dict.setdefault("five", 6)  # filled_dict["five"] is still 5

我們可以使用update方法用另外一個dict來更新當前dict，比如a.update(b)。對於a和b交集的key會被b覆蓋，a當中不存在的key會被插入進來：

# Adding to a dictionary
filled_dict.update({"four":4})  # => {"one": 1, "two": 2, "three": 3, "four": 4}
filled_dict["four"] = 4         # another way to add to dict

我們一樣可以使用del刪除dict當中的元素，同樣只能傳入key。

Python3.5以上的版本支援使用**來解壓一個dict：

{'a': 1, **{'b': 2}}  # => {'a': 1, 'b': 2}
{'a': 1, **{'a': 2}}  # => {'a': 2}

set

set是用來儲存不重複元素的容器，當中的元素都是不同的，相同的元素會被刪除。我們可以通過set()，或者通過{}來進行初始化。注意當我們使用{}的時候，必須要傳入資料，否則Python會將它和dict弄混。

# Sets store ... well sets
empty_set = set()
# Initialize a set with a bunch of values. Yeah, it looks a bit like a dict. Sorry.
some_set = {1, 1, 2, 2, 3, 4}  # some_set is now {1, 2, 3, 4}

set當中的元素也必須是不可變物件，因此list不能傳入set。

# Similar to keys of a dictionary, elements of a set have to be immutable.
invalid_set = {[1], 1}  # => Raises a TypeError: unhashable type: 'list'
valid_set = {(1,), 1}

可以呼叫add方法為set插入元素：

# Add one more item to the set
filled_set = some_set
filled_set.add(5)  # filled_set is now {1, 2, 3, 4, 5}
# Sets do not have duplicate elements
filled_set.add(5)  # it remains as before {1, 2, 3, 4, 5}

set還可以被認為是集合，所以它還支援一些集合交叉並補的操作。

# Do set intersection with &
# 計算交集
other_set = {3, 4, 5, 6}
filled_set & other_set  # => {3, 4, 5}

# Do set union with |
# 計算並集
filled_set | other_set  # => {1, 2, 3, 4, 5, 6}

# Do set difference with -
# 計算差集
{1, 2, 3, 4} - {2, 3, 5}  # => {1, 4}

# Do set symmetric difference with ^
# 這個有點特殊，計算對稱集，也就是去掉重複元素剩下的內容
{1, 2, 3, 4} ^ {2, 3, 5}  # => {1, 4, 5}

set還支援超集和子集的判斷，我們可以用大於等於和小於等於號判斷一個set是不是另一個的超集或子集：

# Check if set on the left is a superset of set on the right
{1, 2} >= {1, 2, 3} # => False

# Check if set on the left is a subset of set on the right
{1, 2} <= {1, 2, 3} # => True

和dict一樣，我們可以使用in判斷元素在不在set當中。用copy可以拷貝一個set。

# Check for existence in a set with in
2 in filled_set   # => True
10 in filled_set  # => False

# Make a one layer deep copy
filled_set = some_set.copy()  # filled_set is {1, 2, 3, 4, 5}
filled_set is some_set        # => False

控制流和迭代

判斷語句

Python當中的判斷語句非常簡單，並且Python不支援switch，所以即使是多個條件，我們也只能羅列if-else。

# Let's just make a variable
some_var = 5

# Here is an if statement. Indentation is significant in Python!
# Convention is to use four spaces, not tabs.
# This prints "some_var is smaller than 10"
if some_var > 10:
    print("some_var is totally bigger than 10.")
elif some_var < 10:    # This elif clause is optional.
    print("some_var is smaller than 10.")
else:                  # This is optional too.
    print("some_var is indeed 10.")

迴圈

我們可以用in來迴圈迭代一個list當中的內容，這也是Python當中基本的迴圈方式。

"""
For loops iterate over lists
prints:
    dog is a mammal
    cat is a mammal
    mouse is a mammal
"""
for animal in ["dog", "cat", "mouse"]:
    # You can use format() to interpolate formatted strings
    print("{} is a mammal".format(animal))

如果我們要迴圈一個範圍，可以使用range。range加上一個引數表示從0開始的序列，比如range(10)，表示[0, 10)區間內的所有整數：

"""
"range(number)" returns an iterable of numbers
from zero to the given number
prints:
    0
    1
    2
    3
"""
for i in range(4):
    print(i)

如果我們傳入兩個引數，則代表迭代區間的首尾。

"""
"range(lower, upper)" returns an iterable of numbers
from the lower number to the upper number
prints:
    4
    5
    6
    7
"""
for i in range(4, 8):
    print(i)

如果我們傳入第三個元素，表示每次迴圈變數自增的步長。

"""
"range(lower, upper, step)" returns an iterable of numbers
from the lower number to the upper number, while incrementing
by step. If step is not indicated, the default value is 1.
prints:
    4
    6
"""
for i in range(4, 8, 2):
    print(i)

如果使用enumerate函式，可以同時迭代一個list的下標和元素：

"""
To loop over a list, and retrieve both the index and the value of each item in the list
prints:
    0 dog
    1 cat
    2 mouse
"""
animals = ["dog", "cat", "mouse"]
for i, value in enumerate(animals):
    print(i, value)

while迴圈和C++類似，當條件為True時執行，為false時退出。並且判斷條件不需要加上括號：

"""
While loops go until a condition is no longer met.
prints:
    0
    1
    2
    3
"""
x = 0
while x < 4:
    print(x)
    x += 1  # Shorthand for x = x + 1

捕獲異常

Python當中使用try和except捕獲異常，我們可以在except後面限制異常的型別。如果有多個型別可以寫多個except，還可以使用else語句表示其他所有的型別。finally語句內的語法無論是否會觸發異常都必定執行：

# Handle exceptions with a try/except block
try:
    # Use "raise" to raise an error
    raise IndexError("This is an index error")
except IndexError as e:
    pass                 # Pass is just a no-op. Usually you would do recovery here.
except (TypeError, NameError):
    pass                 # Multiple exceptions can be handled together, if required.
else:                    # Optional clause to the try/except block. Must follow all except blocks
    print("All good!")   # Runs only if the code in try raises no exceptions
finally:                 #  Execute under all circumstances
    print("We can clean up resources here")

with操作

在Python當中我們經常會使用資源，最常見的就是open開啟一個檔案。我們打開了檔案控制代碼就一定要關閉，但是如果我們手動來編碼，經常會忘記執行close操作。並且如果檔案異常，還會觸發異常。這個時候我們可以使用with語句來代替這部分處理，使用with會自動在with塊執行結束或者是觸發異常時關閉開啟的資源。

以下是with的幾種用法和功能：

# Instead of try/finally to cleanup resources you can use a with statement
# 代替使用try/finally語句來關閉資源
with open("myfile.txt") as f:
    for line in f:
        print(line)

# Writing to a file
# 使用with寫入檔案
contents = {"aa": 12, "bb": 21}
with open("myfile1.txt", "w+") as file:
    file.write(str(contents))        # writes a string to a file

with open("myfile2.txt", "w+") as file:
    file.write(json.dumps(contents)) # writes an object to a file

# Reading from a file
# 使用with讀取檔案
with open('myfile1.txt', "r+") as file:
    contents = file.read()           # reads a string from a file
print(contents)
# print: {"aa": 12, "bb": 21}

with open('myfile2.txt', "r+") as file:
    contents = json.load(file)       # reads a json object from a file
print(contents)     
# print: {"aa": 12, "bb": 21}

可迭代物件

凡是可以使用in語句來迭代的物件都叫做可迭代物件，它和迭代器不是一個含義。這裡只有可迭代物件的介紹，想要了解迭代器的具體內容，請移步傳送門：

Python——五分鐘帶你弄懂迭代器與生成器，夯實程式碼能力

當我們呼叫dict當中的keys方法的時候，返回的結果就是一個可迭代物件。

# Python offers a fundamental abstraction called the Iterable.
# An iterable is an object that can be treated as a sequence.
# The object returned by the range function, is an iterable.

filled_dict = {"one": 1, "two": 2, "three": 3}
our_iterable = filled_dict.keys()
print(our_iterable)  # => dict_keys(['one', 'two', 'three']). This is an object that implements our Iterable interface.

# We can loop over it.
for i in our_iterable:
    print(i)  # Prints one, two, three

我們不能使用下標來訪問可迭代物件，但我們可以用iter將它轉化成迭代器，使用next關鍵字來獲取下一個元素。也可以將它轉化成list型別，變成一個list。

# However we cannot address elements by index.
our_iterable[1]  # Raises a TypeError

# An iterable is an object that knows how to create an iterator.
our_iterator = iter(our_iterable)

# Our iterator is an object that can remember the state as we traverse through it.
# We get the next object with "next()".
next(our_iterator)  # => "one"

# It maintains state as we iterate.
next(our_iterator)  # => "two"
next(our_iterator)  # => "three"

# After the iterator has returned all of its data, it raises a StopIteration exception
next(our_iterator)  # Raises StopIteration

# We can also loop over it, in fact, "for" does this implicitly!
our_iterator = iter(our_iterable)
for i in our_iterator:
    print(i)  # Prints one, two, three

# You can grab all the elements of an iterable or iterator by calling list() on it.
list(our_iterable)  # => Returns ["one", "two", "three"]
list(our_iterator)  # => Returns [] because state is saved

函式

使用def關鍵字來定義函式，我們在傳參的時候如果指定函式內的引數名，可以不按照函式定義的順序傳參：

# Use "def" to create new functions
def add(x, y):
    print("x is {} and y is {}".format(x, y))
    return x + y  # Return values with a return statement

# Calling functions with parameters
add(5, 6)  # => prints out "x is 5 and y is 6" and returns 11

# Another way to call functions is with keyword arguments
add(y=6, x=5)  # Keyword arguments can arrive in any order.

可以在引數名之前加上*表示任意長度的引數，引數會被轉化成list：

# You can define functions that take a variable number of
# positional arguments
def varargs(*args):
    return args

varargs(1, 2, 3